Most bicycle helmets available have (a) a thin outer layer, which may be made, for example, out of polypropylene that is able to absorb initial peak impact forces, (b) a shell within the thin layer and composed of expanded polystyrene that absorbs both initial and subsequent impact forces and (c) padding within the expanded polystyrene shell both to provide comfort to the user and to adjust the shape of the internal cavity within the shell for different shaped and sized heads.
In general, a cycling helmet should fit closely over the cyclist's head so that any impact force is spread over as wide an area of the head as possible. The impact forces are absorbed by the thin polypropylene layer and the expanded polystyrene shell. In addition, some helmets fracture under impact, which also absorbs energy and reduces the energy transferred to the head.
Cycling helmets are often treated roughly and such rough treatment can impair the effectiveness of the helmet. However, there is often no outward visible sign of such impairment.
As mentioned, cycling helmets and helmets for other uses are generally made of synthetic plastics. Although it would be desirable to make the helmets at least partly out of natural material that could be recycled, it is counter-intuitive to use such materials in applications requiring the resistance of such strong forces.
Helmets should generally be light to be acceptable to wearers. Sports protective helmets should also be well ventilated so that sweat does not accumulate around the user's head and so that body heat generated due to the exertion of cycling or other sport can be displaced through the head.
Although the materials used for making the cycling helmets are not particularly expensive, it would advantageous to use cheaper materials, if possible.